Summary The Bell206 helicopter, serial number845, registration C-GPOS, with one pilot and two forestry workers aboard, was hovering approximately 150feet above a forestry cut block, about 4000feet above sea level. It was about 0600 Pacific daylight time, the sky was clear, the winds were light and variable, and the outside air temperature was about 14C. The crew were scanning for hot spots in the cut block when black smoke engulfed the cabin. The pilot turned the helicopter to the right, lowered the collective, and descended in an attempt to reach a road. As he applied collective in a high flare to reduce the helicopter's rate of descent, the pilot sensed a deterioration of power by way of diminishing rotor rpm. He immediately bottomed the collective then raised it in the final stages of the emergency landing. The helicopter landed hard, nose down, short of the road, and the skid landing gear collapsed. The helicopter turned about 180degrees and rolled onto its left side. The engine continued to run and was shut down by the pilot. The smoke was not apparent after the emergency landing and there was no post-impact fire. The pilot and the front seat passenger were trapped in the wreckage for a short time until the back seat passenger was able to assist them. The pilot suffered a back injury and both passengers suffered minor injuries. The helicopter was substantially damaged. Ce rapport est galement disponible en franais. Other Factual Information The wreckage was transported to the Transportation Safety Board of Canada (TSB) facility in Richmond, British Columbia, for examination. External inspection of the engine revealed no anomalies, and both the compressor and power turbine wheels rotated freely. The engine to main rotor transmission coupling was found separated and its grease had flung out, and one of the coupling boots was burnt. Close examination of the coupling indicated that it had been compressed and pulled apart. The main-rotor transmission area revealed damage consistent with the main-rotor transmission rocking fore and aft before deforming its mounting and coming to rest in a forward tilt of approximately 20degrees. The fuel cell was intact and contained about 500pounds of fuel. The fuel valve was selected off, and the throttle was in the closed position. The landing gear skid tubes and forward cross tube were broken and the cross tubes had rotated aft and up. There were marks where the after-market step attached to the cross tubes had come up against the pilot's door, preventing it from opening. Bell206 series helicopters are not normally equipped with doors or windows that have emergency release systems, and this helicopter had none. The bottom of the pilot's seat was deformed slightly, but not enough to allow contact with the seat support honeycomb plate. Photo1. Sulfidation of number1 GPturbine During the examination of the engine, it was installed in an approved engine test facility where a full test was attempted; however, the engine vibrated excessively and the test was aborted. Some smoke was evident during the spool down of the engine. The engine was then disassembled for inspection. Of the 38blades on the number1 gas producer (GP) turbine,29were damaged significantly. One was broken off close to its root, and most of the others were missing pieces. Examination of the remaining fracture surface of the blade that broke close to its root showed indications of a progressive failure (fatigue crack). The other fracture surfaces showed failure by overload. More detailed examination of the GP turbine blades showed extensive sulfidation corrosion1 (seePhoto1). The blade surfaces were flaking and there were fatigue cracks in many of the blades. The turbine first-stage nozzle labyrinth seal was damaged and the outer fibreglass packing was partially missing. There was soot at the joint between the turbine nozzle and number1 GPturbine and thick, black, wet soot around the labyrinth seal and on the turbine coupling. Engine compressor washing is recommended in corrosive and dirty environments. It is also prescribed when engine performance has depreciated, as noted from power assurance checks. However, there has been concern that some tap water can in itself be corrosive. Operators rarely do compressor washes when their helicopters are operating away from a salt environment. They will wash compressors when they can see dirt building up at the air intake, or when an engine is performing below specifications. It is not clear if compressor washing effectively cleans the GP turbine wheel. On 10February2000, Rolls-Royce Allison issued a Commercial Service Letter, Hot Corrosion-Sulfidation, describing hot corrosion (sulfidation) and suggesting that operators look for it. However, no periodic maintenance inspection was directed by Transport Canada (TC) and none was implemented by the operator with regard to sulfidation corrosion. A review of the helicopter's log books and maintenance documentation revealed that the helicopter was maintained in accordance with the standards approved by TC, and that there was no requirement for the company to comply with the manufacturer's recommendations regarding maintenance. The GP turbine had reached 1374.4hours since new, which is 400.6hours before reaching its time for replacement. Documented engine power assurance checks, after the turbine was installed, show the engine to be performing above specifications. However, there were no power assurance checks documented just before the accident, and there were no compressor washes documented, nor were either required by regulation. During the time this turbine was installed, this helicopter operated primarily in what is perceived to be a non-corrosive environment (dry, sparsely populated, inland, etc.) However, sulfidation can be triggered by corrosive substances found in most environments, such as smoke. The number1 GPturbine wheel had been installed with about 200hours of previous use in an unknown environment. Operating a single-engine helicopter at 150feet above ground at a slow airspeed or hovering compromises the pilot's ability to complete an emergency landing successfully in the event of an engine power loss. The helicopter flight manual includes a height /velocity chart that documents this risk. The risk is enhanced by increased helicopter gross weight and high density altitude. At the time of the accident, the helicopter was being operated at a density altitude of approximately 4500feet and at a gross weight of approximately 3000pounds, within the limitations set out by the manufacturer. The maximum gross weight of the helicopter is 3200pounds. The pilot's experience on the helicopter type was extensive. His training and certifications were current and appropriate for the operation.